On-board diagnostics for an opposed-piston engine equipped with a supercharger

ABSTRACT

On-board diagnostic monitoring of a two-stroke cycle, opposed-piston engine includes diagnostic monitoring of an air handling system equipped with a supercharger to determine whether the supercharger is functioning properly.

RELATED APPLICATIONS

This application contains subject matter related to the subject matterof commonly owned U.S. application Ser. No. 13/926,360, filed Jun. 25,2013; commonly-owned U.S. application Ser. No. 13/974,883, filed Aug.23, 2013; and, commonly-owned U.S. application Ser. No. 13/974,935,filed Aug. 23, 2013.

BACKGROUND

The field is internal combustion engines, particularly two-stroke cycle,opposed-piston engines. More specifically, the field is related toon-board diagnostic monitoring of the air handling systems ofopposed-piston engines equipped with superchargers to determine whetherthe superchargers are functioning properly. The field also includesdiagnostic monitoring of opposed-piston air handling elements associatedwith supercharger operations.

A two-stroke cycle engine is an internal combustion engine thatcompletes a cycle of operation with a single complete rotation of acrankshaft and two strokes of a piston connected to the crankshaft. Thestrokes are typically denoted as compression and power strokes. Oneexample of a two-stroke cycle engine is an opposed-piston engine inwhich two pistons are disposed in the bore of a cylinder forreciprocating movement in opposing directions along the central axis ofthe cylinder. Each piston moves between a bottom center (BC) locationwhere it is nearest one end of the cylinder and a top center (TC)location where it is furthest from the one end. The cylinder has portsformed in the cylinder sidewall near respective BC piston locations.Each of the opposed pistons controls one of the ports, opening the portas it moves to its BC location, and closing the port as it moves from BCtoward its TC location. One of the ports serves to admit charge air(sometimes called “scavenging air”) into the bore, the other providespassage for the products of combustion out of the bore; these arerespectively termed “intake” and “exhaust” ports (in some descriptions,intake ports are referred to as “air” ports or “scavenge” ports). In auniflow-scavenged opposed-piston engine, pressurized charge air enters acylinder through its intake port as exhaust gas flows out of its exhaustport, thus gas flows through the cylinder in a single direction(“uniflow”)—from intake port to exhaust port.

With reference to FIG. 1, a two-stroke cycle internal combustion engineis embodied in an opposed-piston engine 10 having at least one portedcylinder 50. For example, the engine may have one ported cylinder, twoported cylinders, three ported cylinders, or four or more portedcylinders. Each ported cylinder 50 has a bore 52 and longitudinallyspaced intake and exhaust ports 54 and 56 formed or machined inrespective ends of a cylinder wall. Each of the intake and exhaust ports54 and 56 includes one or more circumferential arrays of openings inwhich adjacent openings are separated by a solid bridge. In somedescriptions, each opening is referred to as a “port”; however, theconstruction of a circumferential array of such “ports” is no differentthan the port constructions shown in FIG. 1. Pistons 60 and 62 areslideably disposed in the bore 52 of each cylinder with their endsurfaces 61 and 63 opposing one another. Movements of the pistons 60control the operations of the intake ports 54. Movements of the pistons62 control the operations of the exhaust ports 56. Thus, the ports 54and 56 are referred to as “piston controlled ports”. Pistons 60controlling the exhaust ports (“exhaust pistons”) are coupled to acrankshaft 71. Pistons 62 controlling the intake ports of the engine(“intake ports”) are coupled to a crankshaft 72.

As pistons 60 and 62 approach respective TC locations, a combustionchamber is defined in the bore 52 between the end surfaces 61 and 63.Fuel is injected directly into the combustion chamber through at leastone fuel injector nozzle 70 positioned in an opening through thesidewall of a cylinder 50. The fuel mixes with charge air admittedthrough the intake port 54. As the mixture is compressed between the endsurfaces it reaches a temperature that causes the fuel to ignite; insome instances, ignition may be assisted, as by spark or glow plugs.Combustion follows.

The engine 10 includes an air handling system 80 that manages thetransport of charge air provided to, and exhaust gas produced by, theengine 10. A representative air handling system construction includes acharge air subsystem and an exhaust subsystem. The charge air subsystemreceives and compresses air and includes a charge air channel thatdelivers the compressed air to the intake port or ports of the engine.One or more stages of compression may be provided. For example, thecharge air subsystem may comprise one or both of a turbine-drivencompressor and a supercharger. The charge air channel typically includesat least one air cooler that is coupled to receive and cool the chargeair (or a mixture of gasses including charge air) before delivery to theintake port or ports of the engine. The exhaust subsystem includes anexhaust channel that transports exhaust products from exhaust ports ofthe engine for delivery to other exhaust components.

A typical air handling system for an opposed-piston engine is shown inFIG. 1. The air handling system 80 may comprise a turbocharger 120 witha turbine 121 and a compressor 122 that rotate on a common shaft 123.The turbine 121 is coupled to the exhaust subsystem and the compressor122 is coupled to the charge air subsystem. The turbocharger 120extracts energy from exhaust gas that exits the exhaust ports 56 andflows into an exhaust channel 124 directly from the exhaust port orports 56, or from an exhaust manifold 125 or an exhaust plenum thatcollects exhaust gasses output through the exhaust port or ports 56. Inthis regard, the turbine 121 is rotated by exhaust gas passing throughit. This rotates the compressor 122, causing it to generate charge airby compressing fresh air. Charge air output by the compressor 122 flowsthrough a charge air channel 126 to a cooler 127 whence it is pumped bya supercharger 110 to the intake ports. Charge air compressed by thesupercharger 110 can be output through a cooler 129 to an intakemanifold 130 or an intake plenum for provision to the intake port orports 54. In some instances, exhaust products may be recirculated intothe charge air channel through an exhaust gas recirculation (EGR)channel 131 for the purpose of reducing unwanted emissions.

The opposed-piston engine is provided with an engine controlmechanization—a computer-based system including one or more electroniccontrol units coupled to associated sensors, actuators, and othermachinery throughout the engine that governs the operations of variousengine systems, including the air handling system, a fuel system, acooling system, and other systems. The engine control elements thatgovern the air handling system are referred to collectively as the “airhandling control mechanization”.

In an air handling system for a two-stroke cycle, opposed-piston engine,the supercharger performs a number of important functions. For example,it provides a positive charge air pressure to drive uniflow scavengingthrough the cylinders. In addition, the supercharger delivers boost(increased air pressure) when the engine accelerates. Further, thesupercharger may be employed to pump recirculated exhaust productsthrough the EGR channel. In many instances, the supercharger is one ofthe key components of the air handling system in an opposed-pistonengine. Deterioration of supercharger performance can have significantnegative impact on the emissions, general performance, and durability ofthe engine.

Manifestly, it is important to monitor and diagnose the performance ofthe supercharger and provide clear indications when its performancefalls below an acceptable limit. Accordingly, there is a need for airhandling control mechanizations for opposed-piston engines that arecapable of confirming that a supercharger is operating correctly, anddiagnosing and reporting faults that may occur in superchargeroperation.

Optimal operation of a supercharger may require an additional elementthat enables modulation of the pressure of charge air output by thesupercharger. In this regard, it is frequently the case that thesupercharger's impeller is coupled to a crankshaft by a direct driveelement such that the impeller cannot rotate independently of thecrankshaft. The fixed relationship between supercharger and crankshaftaffords a rigid and imprecise regulation of boost (charge air compressedby the supercharger). In order to modulate boost pressure and gaingreater precision in charge air handling under such conditions, a bypassvalve is provided in fluid communication with the charge air channeldownstream of the supercharger outlet to adjust the pressure of boostair produced by the supercharger as needed in response to engineoperation. In other instances, the supercharger may be driven by avariable speed device with a transmission that enables thesupercharger's speed to be controlled independently of the crankshaft.

In some instances, an apparent deterioration of supercharger performancecan result from faulty performance of an element that controls ormodulates the supercharging operation. For example, a stickysupercharger bypass valve or a faulty bypass valve actuator can impairthe air handling system's boost response in a way that isindistinguishable from the performance of a supercharger with adefective belt. In another instance, a variable speed supercharger drivewith a deteriorating transmission can cause boost pressure fluctuation,as would happen in the case of a supercharger with compromised rotors.

Manifestly, it is important to continuously monitor and diagnose theperformances of elements that control or modulate superchargeroperations, and to take appropriate actions if performance of such anelement falls below an acceptable limit. Accordingly, there is a needfor air handling control mechanizations for opposed-piston engines thatare capable of confirming that air handling system elements that affectsupercharger operations are operating correctly, and diagnosing andreporting faults that may occur in their operations.

SUMMARY

It is therefore advantageous to incorporate diagnostics into the controlmechanization of an air handling system for an opposed-piston engineequipped with a supercharger that monitor the supercharger and detectfaults that would keep it from delivering a desired supply of compressedcharge air to the intake ports of the engine.

It is further desirable to invest such diagnostics with the capabilityof monitoring elements that control or modulate supercharger operationsand detecting faults in those elements that would keep the superchargerfrom delivering a desired supply of compressed charge air to the engine.

In some instances it is desirable to incorporate diagnostics into thecontrol mechanization of an air handling system for an opposed-pistonengine that monitor the supercharger, as well as one or both of asupercharger bypass valve and a supercharger variable speed drive, todetect faults that would keep the supercharger from delivering a desiredsupply of compressed charge air to the intake ports of the engine.

A two-stroke cycle, opposed-piston engine is equipped with an enginecontrol mechanization that monitors and governs engine operations,including the operation of an air handling system with a supercharger.According to one aspect, the air handling control mechanism includes adiagnostic system to confirm that the supercharger is operatingcorrectly. The diagnostic system diagnoses and reports faults that mayoccur in supercharger operation. Preferably, the diagnostic system is anon-board diagnostics arrangement.

According to another aspect, an air handling control mechanism executesa diagnostic method that monitors supercharger operation and reportsfaults that would keep the supercharger from delivering a desired supplyof pressurized charge air to the intake ports of an opposed-pistonengine. Preferably, the diagnostic method is performed by an on-boarddiagnostics system.

According to yet another aspect, the air handling control mechanismincludes a diagnostic system and methods performed thereby to confirmthat the supercharger and one of a supercharger bypass valve and asupercharger variable speed drive are operating correctly. Preferably,the diagnostic system is an on-board diagnostics arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

The below-described drawings are meant to illustrate principles andexamples discussed in the following disclosure. They are not necessarilyto any scale.

FIG. 1 is a diagram of an opposed-piston engine equipped with an airhandling system and is properly labeled “Prior Art”.

FIG. 2 is a schematic drawing of an air handling system of a two-strokecycle, opposed-piston engine equipped with a control mechanization thatincludes a diagnostic system for monitoring the operation of asupercharger with which the air handling system is equipped withon-board diagnostics capability.

FIG. 3 is a schematic illustration of an on-board diagnostics systemthat can be used with the air handling system of FIG. 2.

FIG. 4 is a flow diagram of an on-board diagnostics process that can beused with the air handling system of FIG. 2.

FIG. 5. is a schematic illustration of a table that can be used inon-board diagnostics for the air-handling system of FIG. 2 to relateon-board diagnostics measurements and/or estimates to expected responsesby elements being monitored.

FIG. 6 is a flow diagram of an on-board supercharger diagnostic processthat can be used with the air handling system of FIG. 2.

FIG. 7 is a flow diagram of an on-board supercharger bypass valvediagnostic process that can be used with the air handling system of FIG.2.

FIG. 8 is a diagram of a first on-board supercharger variable speeddrive diagnostic process that can be used with the air handling systemof FIG. 2.

FIG. 9 is a diagram of a second on-board supercharger variable speeddrive diagnostic process that can be used with the air handling systemof FIG. 2.

DETAILED DESCRIPTION

Opposed-Piston Engine Air Handling System:

An air handling system 200 for a two-stroke cycle, opposed-piston engine201 such as the engine illustrated by FIG. 1 is shown in schematic formin FIG. 2. The air handling system 200 includes a supercharger 210 whichreceives input rotary power from a drive unit 212. Although the figureshows the drive unit 212 separately from the supercharger 210, this isnot meant to exclude the option of integrating these elements into asingle unit. The supercharger 210 includes an inlet 213 and an outlet214.

Preferably, but not necessarily, the air handling system 200 alsoincludes a turbocharger 220 with a turbine 221 and a compressor 222. Theturbine 221 is coupled to an exhaust channel 224 and the compressor 222is coupled to a charge air channel 225. The turbine 221 is spun byexhaust gas expelled from the exhaust ports 156 of the engine andtransported through the exhaust channel 224. This spins the compressor222, causing it to generate charge air by compressing inlet air thatflows into the charge air channel. Compressed charge air output by thecompressor 222 is transported through the charge air channel 225 to acooler 227. In this configuration, the supercharger 210 constitutes asecond stage of compression in the air handling system 200 (followingthe compressor 222). In any case, the supercharger 210 compresses air inthe charge air channel and provides compressed charge air (sometimescalled “boost”) to the intake ports 154 of the opposed-piston engine. Insome instances, a cooler 229 may be provided to cool the output of thesupercharger 210. Optionally, the air handling system may include an EGRbranch 230 to transport exhaust products from the exhaust channel 224 tothe charge air channel 225 via an EGR mixer 226.

Control of the gas transport configuration of the air handling system isimplemented by a mechanization that includes an ECU (engine controlunit) 240, air handling processes executed on the ECU, air handlingvalves and associated actuators, the supercharger 210, and enginesensors. Air handling system control is exercised by settings ofvariable valves. In this regard, for example, a supercharger bypassvalve 231 bleeds charge air produced by the supercharger 210 through abypass channel 232 so as to modulate charge air pressure, and dampensurges, at the intake ports 154. An EGR valve 233 adjusts the amount ofexhaust gas that is transported through the EGR branch 230 to the chargeair channel 225 for control of emissions. A wastegate valve 235 shuntsexhaust gas around the turbine 221 in order to protect turbochargercomponents against pressure surges in the air handling system. Abackpressure valve 237 regulates exhaust pressure at the turbine outletin order to warm the engine quickly during start-up. For fast, preciseautomatic operation, it is preferred that these and other valves in theair handling system be high-speed, computer-controlled devices, withcontinuously-variable settings. The ECU 240 is in control communicationwith actuators (not seen) that operate the valves in response toECU-issued control signals.

The ECU 240 monitors air handling system operating conditions by way ofvarious air handling sensors. In this regard, for example, superchargerintake and outlet charge air pressures are measured with gas pressuresensors 252 and 254, respectively. Air mass flow into the charge airsubsystem is measured by sensor 255; exhaust mass flow in the EGRchannel is measured by sensor 256; and gas temperature at thesupercharger inlet 213 is measured by gas temperature sensor 257. Forpurposes of this specification these and other sensors may comprisephysical measurement instruments and/or virtual systems. The sensorpositions shown in the figures are indicative of locations where in theair handling system the measured parameter value could be obtained ifmeasured by a physical instrument.

In most cases, to obtain the rotary power necessary to its operation,the supercharger 210 is directly coupled to the engine-usually via acrankshaft-driven drive apparatus. In these cases the speed of thesupercharger is dependent on the speed of the engine. In some instances,it is desirable to be able to vary the supercharger's speedindependently of engine speed so as to gain greater flexibility andprecision in charge air control, which can improve the air handlingoperations and contribute to the achievement of optimal engineperformance. For example, at low engine speeds when quick accelerationis required, faster rotation of the supercharger delivers higher boostlevels than would be available with conventional coupling to an enginecrankshaft. Thus, it may be the case that the drive 212 is equipped witha transmission that enables the supercharger to be driven, under commandof the air handling control mechanization, at a continuously-, orincrementally-, variable speed, independently of a crankshaft. In someof these instances, the supercharger bypass valve 231 may be redundant.That is to say, the greater the variability in supercharger speedafforded by the variable-speed drive, the less likely a bypass valvewould be needed to modulate boost pressure.

However, there may be instances wherein a drive unit is constructed toprovide a limited number of speeds (two speeds, for example) andflexibility in control of boost pressure may require the operations of asupercharger bypass valve. Such instances are addressed in thisdisclosure.

With reference to FIGS. 2 and 3, the ECU 240 executes an air handlingcontrol process to configure the gas transport configuration of the airhandling system as required by a current engine operating state, inresponse to engine conditions measured by various engine sensors. Theair handling control process includes one or more air handling processes242 that are continuously executed during engine operation, and one ormore diagnostic processes 244 that are recurrently executed as theengine operates. To the extent that the engine is assembled and readyfor installation, or already installed, for example in a vehicle or avessel, with the ECU 240 and the control processes it executes,including diagnostics, available or situated on the installation, thecontrols and diagnostics are considered to be “on-board.” The on-boarddiagnostics (“OBD”) include reporting capabilities that provideinformation and/or trouble codes, or visual and/or audible indicationsrelating to the operability or inoperability of engine components andsystems.

The ECU 240 is constructed with a microprocessor, associated programstorage, program memory, and data storage. Code that enables the ECU toconduct various control and diagnostics processes resides in the programstorage. Interface electronics associated with or in the ECU formatinput data signals and generate output control and information signalsand connect the ECU with sensors, actuators, displays, indicators, andother peripheral devices.

The ECU 240 executes various engine system control processes, includingprocesses for control of air handling and fuel injection systems. Suchprocesses may include open- and/or closed-loop air handling processes.These processes use values of control parameters associated withoperation of an opposed-piston engine air handling system, and executevarious procedures to control air handling elements based on the controlparameter values. The ECU 240 may obtain control parameter values by anyone or more of a number of instrumentalities including sensormeasurement, table look-up, calculation, estimation, and programdeclaration. The description of any particular instrumentality of dataobtainment in the following specification is for illustration only andis not intended to exclude, disclaim, or surrender any alternative. TheECU 240 includes registers 245 that receive data signal inputs fromsensors, and registers 247 that store commands which are converted tocontrol signals communicated to actuators. For an example ofopposed-piston air handling control mechanizations of this type, seecommonly-owned U.S. application Ser. No. 13/926,360, filed Jun. 25, 2013for “Air Handling Control for Opposed-Piston Engines with UniflowScavenging.”

According to one aspect of this disclosure, a control mechanization forgoverning an air handling system such as the system 200 of FIG. 2includes an on-board diagnostic system to confirm that the supercharger210 is functioning correctly. The diagnostic system diagnoses andreports faults that may occur in the supercharger and in air handlingelements associated with supercharger performance. As per FIG. 3, thediagnostic system includes the ECU 240, gas pressure sensors 252 and254, and one or more diagnostic processes 244 and associated data tables249 on the ECU 240. Diagnostic results and/or faults are reported by thediagnostic control processes by way of an OBD monitor 262 which causesone or more OBD fault indications to be output. Such fault indicationsmay include one or more of indicator lights and icons, diagnostic codes,and information readouts. The sensor 252 measures the pressure of chargeair in the charge air channel upstream of the supercharger, near thesupercharger inlet 213. The sensor 254 measures the pressure in thecharge air channel downstream of the supercharger, near the superchargeroutlet 214.

On Board Diagnostics for the Supercharger and Related Components:

The construction and operation of the supercharger are conventional. Adrive unit receives a mechanical input drive, typically obtained fromone of the crankshafts, and produces an output drive in responsethereto. The output drive is coupled to the input shaft of thesupercharger and causes one or more elements of the supercharger'scompression mechanism to rotate. As per FIG. 3, air fed at some inputpressure (P_(in)) into the compression mechanism is compressed(pressurized) and the compressed air is output at some output pressure(P_(out)) by the supercharger (SC) 210. The gas pressure sensor 252measures P_(in); the gas pressure sensor 254 measures P_(out). It isuseful to control air handling system operation by varying the rate offlow of compressed charge air output by the supercharger in order tooptimize engine performance throughout its operating range. Charge airflow can be varied by way of the bypass channel 232 whose input is influid communication with the charge air channel 225, downstream of thesupercharger outlet 214. Provision of the bypass valve 231 in the bypasschannel 232 enables modulation of charge air pressure downstream of thesupercharger outlet 214.

With reference to FIGS. 2, 3, and 4, an initial air handling diagnosticprocess 400 determines if engine air handling performance is withinexpected parameter values. Specifically, the process 400 monitors andevaluates supercharger performance; in some instances, the process alsomonitors and evaluates performances of an air handling elementimmediately associated with supercharger operation, such as the bypassvalve 231 and/or the drive 212. The process 400 begins at state 402 bydetermining operational validity of air handling sensors. In state 402,sensor operations are checked for air mass flow into the charge airsubsystem (sensor 255), exhaust mass flow in the EGR channel (sensor256), and supercharger intake and outlet pressures (sensors 252 and 254,respectively); sensor operations for other measurements are alsochecked. A check is made in state 404 to determine if the airflowsensors are producing valid data signals. A sensor failure stops thediagnostic process 400 at state 410 since any further diagnostics wouldbe invalid. If the sensor data signals are determined to be valid, oneor more of on board supercharger diagnostic, supercharger bypass valvediagnostic, and supercharger variable speed drive diagnostic processesare executed in state 406 to determine if these air handling systemelements are functioning within respective established operatingparameter values. These OBD processes may be executed in some preferredsequence, simultaneously, or in overlapping fashion. Further, these OBDprocesses may comprise separate or interleaved routines. In state 408,if these elements are operating within respective design parameters, thediagnostic process 400 is completed and stopped in state 410. If,however, any diagnostic process indicates a fault, the OBD monitor istriggered in state 412, which causes output of an OBD fault indicationin state 413. If the fault, or air handling system failure, is severeenough, the air handling control system may shut down the engine.

Supercharger Diagnostic Process:

With reference to FIGS. 2 and 3, and presuming that the engine isequipped with a turbocharger and an EGR branch, the mixer 226 mixescharge air coming out of the compressor 222 with the recirculatedexhaust. Thus, with EGR, the mass flow of gas into the supercharger 210includes mass air flow into the charge air channel, measured by sensor255, and mass flow of recirculated exhaust through the EGR channel,measured by sensor 256. The mixture is further compressed by thesupercharger 210. If the supercharger performance drops below anacceptable limit then the air handling system won't be able to providethe desired amount of charge to the engine, and thus may fail to meetperformance requirements. Hence, it is useful to provide an OBD processto evaluate operation of the supercharger.

Referring now to FIG. 3, a volumetric flow of gas (charge air) throughthe supercharger 210 is a function of supercharger speed (RPM_(SC)) andthe pressure ratio (P_(out)/P_(in)) across the supercharger 210. Thisrelationship can be mapped for a new supercharger and stored in the ECU240 as a two-dimensional lookup table (SC Performance Table) having aform illustrated in FIG. 5, which is indexed by values of first andsecond parameters (respectively PAR 1 and PAR 2). For the superchargerdiagnostic process, values of volumetric flow through the superchargerin the SC Performance Table are indexed by supercharger speed (RPM_(SC))and pressure ratio (P_(out)/P_(in)). The speed of the supercharger canbe obtained conventionally by way of a speed sensor (not shown) from thesource of the supercharger input drive, such as a crankshaft, or avariable speed drive. A first supercharger mass flow value (W_(sc1), inkg/s) can then be calculated by multiplying a supercharger inlet densitycalculated from supercharger inlet pressure (P_(in)) and superchargertemperature (T_(in), measured by temperature sensor 257) with the valueof supercharger volumetric flow obtained from the SC Performance Table.With the supercharger bypass valve 231 closed, a second superchargermass flow value (W_(sc2), in kg/s) is obtained. The second mass flowvalue is based on the mass flow of air into the air handling system(W_(air), in kg/s), which is measured by the mass air flow sensor. Ifthe air handling system does not include EGR, W_(sc2)=W_(air). If theair handling system includes EGR, then mass flow into the superchargeris obtained by adding the outputs of the mass airflow sensor and the EGRmass flow sensor (W_(egr), in kg/s); thus, W_(sc2)=W_(air)+W_(egr). Whenthe supercharger bypass valve 231 is fully closed, the firstsupercharger mass flow value W_(sc1) should be within a calibrated rangeof the second supercharger mass flow value W_(sc2). If the performanceof the supercharger drops then the flow obtained with the aid of thesupercharger map will be higher than the total mass flow calculatedbased on W_(air). As the performance of the supercharger deterioratesthis difference will increase until a predefined limit L₁ is reached.

FIG. 6 illustrates one embodiment of an on-board supercharger diagnosticprocess 600 that can be used with the air handling system of FIG. 2and/or during state 406 of process 400. In state 602, the setting of thesupercharger bypass valve 231 is determined. If the valve is closed, theprocess transitions to state 604; if not, the valve is closed in state606 and the process transitions to state 604. In state 604, the massflow value W_(SC1) is determined, and then the second mass flow valueW_(SC2) is determined in state 608. The process 600 then transitions tostate 610, where a comparison value is obtained and a decision is thenmade as to whether the comparison value meets a mass flow performancemeasure defined by the predetermined performance limit L₁, as determinedby (W_(sc1)−W_(sc2)>L₁). If the limit L₁ is not exceeded, thesupercharger is performing within specifications and, in state 612, thediagnostic process correlates the comparison value with the diagnosticconclusion that the supercharger is operable to provide compressedcharge air to the piston-controlled intake ports of the opposed-pistonengine. The process then transitions to state 614 where another OBDprocess is run. If the limit is exceeded, then the superchargerperformance is deteriorating and the diagnostic process, in state 616,correlates the comparison value with the diagnostic conclusion that thesupercharger is not operable to provide compressed charge air to thepiston-controlled intake ports of the opposed-piston engine and theprocess 600 transitions to state 614, at which event, the OBD monitor istriggered and a supercharger performance fault indication is output. Theprocess stops in state 618.

From state 614 the ECU 240 executes another diagnostic process accordingto an overall air handling control scheme. For example, the ECU 240 mayexecute a supercharger bypass valve diagnostic process and/or asupercharger variable-speed drive diagnostic process.

Supercharger Bypass Valve Diagnostic Process:

With reference to FIGS. 2 and 3, the boost in charge air pressureprovided by the supercharger 210 to the intake ports 154 is controlledby adjusting the opening (position) of the supercharger bypass valve231. Preferably, the supercharger bypass valve 231 is operated by anelectro-mechanical actuator 234 under control of the ECU 240. In thisregard, the ECU 240 generates a bypass valve set command (BV_SET) thatis converted by interface electronics to a control signal communicatedto the actuator 234. For example, the control signal may comprise apulse-width modulated (PWM) signal that causes the actuator 234 to setthe bypass valve 231 to a position in a range between a fully closedposition where no charge air passes through the valve and fully openposition. Preferably, a position sensor 236 generates a signalindicating the valve's current setting. If the bypass valve performancedrops below an acceptable limit then the air handling system will beunable to provide the desired amount of charge air and recirculatedexhaust to the engine, and thus may fail to meet emission requirements.Consequently, it is useful to provide an OBD process to evaluateoperation of the supercharger bypass valve.

In the following discussions, the air handling parameters arerepresented by the following notations (in which “SC” denotes thesupercharger):

-   W_(SC) _(_) _(valve)=Mass flow rate through bypass valve (kg/s)-   W_(SC) _(_) _(table)=Mass flow rate through the SC estimated from SC    Performance Table (kg/s)-   W_(air)=Mass flow rate of fresh air into the air handling system    (kg/s)-   W_(egr)=Mass flow rate of EGR into the charge air subsystem (kg/s)-   A_(eff)=Effective flow area of the bypass valve (m²) based on valve    opening-   BV_(set)=Current open setting of the bypass valve-   θ_(valve)=Bypass valve open percentage (current open setting/max    open setting)-   P_(out)=SC outlet pressure (Pa)-   P_(in)=SC inlet pressure (Pa)-   T_(out)=SC inlet temperature (K)-   C_(d)=Discharge coefficient for the bypass valve-   γ=Ratio of constant pressure & constant volume specific heat of    charge going through the SC

The mass flow rate of charge air through the supercharger W_(SC) isdetermined with the use of volumetric flow value obtained from the SCPerformance Table as set forth in the description of the superchargerdiagnostic process. For the bypass valve diagnostic process, thisparameter value is denoted as W_(SC) _(_) _(table). When the bypassvalve is in use, i.e. it is not fully closed, then the mass flow throughthe bypass valve can be calculated as follows:W _(SC) _(_) _(valve) =W _(SC) _(_) _(table) −W _(air)(without EGR); W_(SC) _(_) _(valve) =W _(SC) _(_) _(table) −W _(air) −W _(egr) (withEGR).

Based on W_(SC) _(_) _(valve), an effective bypass valve flow pathdiameter can be calculated by modeling the valve as an orifice, forexample, by:

$A_{eff} = {{W_{{sc}\;{\_{valve}}}/C_{d}}\frac{P_{{sc}\;\_\;{out}}}{\sqrt{{RT}_{{sc}\;\_\;{out}}}}{\sqrt{\frac{2\gamma}{\gamma - 1}( {( \frac{P_{{sc}\;\_\;{out}}}{P_{{sc}\;{\_{in}}}} )^{\frac{2}{\gamma}} - ( \frac{P_{{sc}\;\_\;{out}}}{P_{{sc}\;{\_{in}}}} )^{\frac{\gamma + 1}{\gamma}}} )}.}}$

The discharge coefficient C_(d) for the bypass valve can be obtainedempirically by experimental testing, and the coefficient value can bestored in ECU memory. During execution of the bypass valve diagnosticprocess, the effective valve area can then be converted into valveposition based on a look up table that maps the valve position toeffective area.θ=_(sc) _(_) _(valve) =f ⁻¹(A _(eff))

The modeled valve position is then compared to measured valve position(BV_(set)) from the sensor 236. If the difference is greater than anacceptable limit then a fault with the bypass valve is detected andappropriate action can be taken to satisfy OBD requirements.

If no fault is detected with use of the sensor 236, then the next stepis to determine degradation, if any, of the bypass valve operation (forexample, due to a sticky valve). In order to accomplish the task, thecommanded control signal valve (PWM pulse width, for example) isconverted into an expected valve position based on table look up, asfollows:θ_(sc) _(_) _(valve) =f(SC_Valve_PWM)

If the estimated valve position differs from measured valve position bymore than an acceptable limit then a fault with the bypass valve can beset and appropriate action can be taken.

It should be noted that the bypass valve actuator 234 may have currentmeasurement capability (internal to the ECU 240), in which case thebypass valve position can be estimated based on current drawn by theactuator 234. This would effectively detect supercharger-related faultsresulting from a faulty actuator and/or wiring defects.

FIG. 7 illustrates one embodiment of an on-board supercharger bypassvalve diagnostic process 700 that can be used with the air handlingsystem of FIG. 2 and/or during state 406 of process 400. The process 700starts at state 702 and proceeds to state 704 wherein an estimate of thebypass valve mass flow rate W_(SC) _(_) _(valve) is made. Then, in state706 a first bypass valve position based upon A_(eff) is estimated. Instate 708, a second bypass valve position is determined from themeasurement provided by the sensor 236. In state 709, the first bypassvalve position and the second bypass valve position are compared toestablish a bypass valve position comparison value and a decision isthen made as to whether the comparison value meets a bypass valveposition measure defined by a predetermined performance limit L₂. If thelimit L₂ is not exceeded, the bypass valve position sensor is performingwithin specifications and the diagnostic process in state 710 correlatesthe comparison value with the diagnostic conclusion that the bypassvalve position sensor 236 is operable to provide the bypass valveposition. The process then continues to state 712. Otherwise, if thebypass valve position comparison value exceeds L₂, the process 700, instate 714 determines that the bypass valve position sensor 236 is faultyand correlates the comparison value with the diagnostic conclusion thatthe bypass valve position sensor is not operable to provide the bypassvalve position. The OBD monitor is triggered and a supercharger bypassvalve performance fault indication is output. The process 700 then stopsin state 716.

If the process 700 transitions to state 712, a third bypass valveposition is determined based upon an actuator control signal (forexample, a PWM signal) or the current drawn by the bypass valve actuator234. In state 718, the third bypass valve position and a fourth bypassvalve position determined from the measurement provided by the sensor236 (the first bypass valve position may be used or a new measurementmay be made) are compared to establish a second bypass valve positioncomparison value. If the comparison value meets a bypass valve positionmeasure defined by a predetermined performance limit L₃ (if the limit L₃is not exceeded), the bypass valve position actuator 234 is performingwithin specifications and, in state 720, the diagnostic process 700correlates the comparison value with the diagnostic conclusion that thebypass valve 234 is operable to shunt charge air from the charge airchannel. The process then ends in state 716. Otherwise, if the secondbypass valve position comparison value exceeds L₃, the process 700, instate 722 determines that the bypass valve 234 is faulty and correlatesthe comparison value with the diagnostic conclusion that the bypassvalve is not operable to shunt charge air from the charge air channel,at which event the OBD monitor is triggered and a supercharger bypassvalve position sensor performance fault indication is output. Theprocess then ends in state 716.

First Variable-Speed Drive Diagnostic Process:

A supercharger may be equipped for independently-variable speedoperation by way of an auxiliary transmission under open- or closed-loopgovernance of the air handling control mechanization. Such anarrangement may include a continuously-variable transmission (CVT), alsocalled a “variator”. The arrangement may also cover a stepwise-variabletransmission, also called a “multi-speed” transmission, one example ofwhich is the dual-speed supercharger of Antonov Automotive TechnologiesLtd. In either case, the drive is referred to as a “variable-speeddrive.”

A failure in operation of the variable-speed drive can significantlyimpact the performance of the engine in respect of output power andemission levels. For example, when the variable-speed drive cannot shiftto a desired high-speed setting, high airflow requirements cannot be metand poor combustion will result, which can lead to the production ofincreased soot and other undesirable exhaust emissions. On the otherhand, when the variable-speed drive cannot lower the speed of thesupercharger under low load conditions, excessive boost can result thatmay cause engine damage. Consequently, it is useful to provide an OBDprocess to evaluate operation of the supercharger variable speed drive.

In operation, a variable-speed drive receives a mechanical input driveat an input speed (say, RPM_(in)) and provides a mechanical output driveat an output speed (say, RPM_(out)). The output drive is coupled to theinput shaft of the supercharger and causes one or more elements of thesupercharger's compression mechanism to rotate, and so the output speedof the drive (RPM_(out)) is effectively the speed of the supercharger.Air fed at some input pressure (P_(in)) into the compression mechanismis compressed (pressurized) and the compressed air is output at someoutput pressure (P_(out)) by the supercharger The pressure ratio(P_(out)/P_(in)) of the output pressure to the input pressure is variedby varying the speed of the supercharger's input shaft. It is useful tocontrol air handling system operation by varying the pressure ratio ofthe supercharger in order to optimize engine performance throughout itsoperating range. This is done by varying the drive ratio(DR=RPM_(out)/RPM_(in)) of the variable-speed drive. Depending on theconstruction of the variable-speed drive, the drive ratio can be variedcontinuously, or in discrete increments.

With reference to FIGS. 2 and 3, in the air handling system 200 shownthe drive 212 may be constructed as a variable-speed drive. Thevariable-speed drive 212 may be driven by an electro-mechanical actuator215 under control of the ECU 240. In this regard, the ECU command(VAR_SP_RAT_SET) is converted by interface electronics to a controlsignal—for example a pulse-width modulated (PWM) signal—that is coupledto the actuator 215. With this control configuration, a drive ratio (DR)of the variable speed drive 212 can be set, thereby enabling variablecontrol of the speed of the supercharger, independent of crankshaftspeed. Although the variable speed drive 212 and actuator 215 are shownas separate elements in the figure, this is not intended to be limitingas they may be integrated into a single unit.

A supercharger variable-speed drive performance diagnostic compares achange in the pressure ratio Δ(P_(out)/P_(in)) across the supercharger210 that occurs in response to a change supercharger speed caused by achange in the drive ratio Δ(DR). The drive ratio is changed by a controlsignal communicated to the variable speed drive 212. If thevariable-speed drive 212 is operating correctly, a change in the driveratio during the diagnostic routine will cause a corresponding change inthe pressure ratio. If the operation of the variable-speed drive 212 isfaulty, a change in the drive ratio will cause little or no change inthe pressure ratio. The pressure ratio may be measured by noting thedifference between the inlet and outlet pressures of the supercharger210, as indicated by the gas pressure sensors 252 and 254 respectively.If it is the case that the air handling system also includes the bypassvalve 231, the valve is held at a predetermined calibratable state as avariable-speed drive diagnostic process is executed.

FIG. 8 illustrates one embodiment of an on-board superchargervariable-speed drive diagnostic process 800 that can be used with theair handling system of FIG. 2 and/or during state 406 of process 400.Preferably, but not necessarily, the process 800 runs while the engineis in an idle mode of operation. The process 800 starts at state 802. Atstate 804, if the engine includes a supercharger bypass valve then theprocess transitions through state 806 where the bypass valve 231 is setto a predetermined calibratable state in which it is not fully open. (Ifthe bypass valve is fully open the pressure ratio across thesupercharger will be nearly equal to 1, for every setting of thevariable speed drive, and no meaningful diagnostic results can beobtained). Of course, states 804 and 806 can be omitted altogether ifthere is no bypass valve, in which case the process 800 would proceeddirectly from state 802 to state 808. In state 808, the variable speeddrive 212 is set to a first drive ratio (DR₁) and a calculation of afirst supercharger pressure ratio PR₁ is made. Then, in state 810, whilemaintaining air handling valve settings (bypass, wastegate, backpressure, EGR, for example), the process 800 sets the variable-speeddrive to a second drive ratio (DR₂), for example a higher drive ratio,and transitions to state 812 where a second supercharger pressure ratioPR₂ is obtained. In state 814, the first pressure ratio PR₁ and thesecond pressure ratio PR₂ are compared to establish a variable-speeddrive comparison value and a decision is then made as to whether thecomparison value meets a variable-speed drive measure defined by apredetermined performance limit L₄. If the limit L₄ is exceeded, thevariable-speed drive is performing within specifications and thediagnostic process 800 in state 816 correlates the comparison value withthe diagnostic conclusion that the variable-speed drive 212 is operableto drive the supercharger 210. The process ends in state 818. Otherwise,if the variable-speed drive comparison value does not exceed L₄, theprocess 800, in state 820 determines that the variable-speed drive 212is faulty and correlates the comparison value with the diagnosticconclusion that the variable-speed drive is not operable to drive thesupercharger. The OBD monitor is triggered and a variable-speed driveperformance fault indication is activated. The process ends in state816.

Second Variable-Speed Drive Diagnostic Process:

In some instances where the air handling system utilizes both asupercharger bypass valve and a variable speed drive to controloperation of the supercharger, a second supercharger variable-speeddrive performance on-board diagnostic may be useful for detecting afault in the actuator 215 of the variable-speed drive 212. Referring toFIG. 3, this diagnostic process presumes the air handling controlmechanization control routines 242 include a closed-loop process thatcontrols the setting of the bypass valve 231. In this regard, during asteady-state engine operating condition the closed-loop processmaintains the pressure ratio across the supercharger 210 at somepredetermined value by means of the bypass valve 231 (BV_SET). When thedrive ratio is changed, the supercharger speed will change. In order tomaintain the same commanded airflow supply to the engine, thesupercharger bypass valve position (BV_SET) will be changed by theclosed-loop process. For example, when going to a higher drive ratio,the bypass valve 231 would be opened more to maintain the same requiredairflow through the engine.

FIG. 9 illustrates a second embodiment of an on-board superchargervariable-speed drive diagnostic process 900 that can be used with theair handling system of FIG. 2 and/or during state 406 of process 400.This embodiment presumes the presence of both a superchargervariable-speed drive and a supercharger bypass valve. The process isinitiated in state 902. In state 904 the engine is running at somesteady state under control of a closed-loop process which maintains adrive ratio at DR₁ and a bypass valve setting (VS) at VS₁. In state 906,the process 900 causes the drive ratio to be changed (preferably,increased) to another value DR₂ by changing the command settingVAR_SP_RAT_SET. In state 908, the closed-loop control process detectsthe change in pressure ratio and attempts to compensate by changing thebypass valve setting to VS₂. In state 910, the process 900 thentransitions to state 910, where a bypass valve setting comparison valueis obtained and a decision is then made as to whether the comparisonvalue meets a bypass valve performance measure defined by thepredetermined performance limit L₅, as determined by (VS₂−VS₁>L₅). Ifthe limit L₅ is exceeded, the supercharger variable-speed drive isperforming within specifications and the diagnostic process correlatesthe comparison value with the diagnostic conclusion that thesupercharger variable-speed drive is operable. The process 900 thenstops at state 914. If the limit is not exceeded, then thevariable-speed drive performance is deteriorating and the diagnosticprocess, in state 916, correlates the comparison value with thediagnostic conclusion that the variable-speed drive is not operable todrive the supercharger, at which event, the OBD monitor is triggered anda variable-speed drive fault indication is output.

Although this disclosure describes particular on-board diagnosticsembodiments for the air handling system of an opposed-piston engine,these embodiments are set forth merely as examples of underlyingprinciples of this disclosure. Thus, the embodiments are not to beconsidered in any limiting sense.

The invention claimed is:
 1. A diagnostic system for an opposed-pistonengine including at least one cylinder with piston-controlled exhaustand intake ports near respective ends of the cylinder, a charge airchannel to provide charge air to at least one intake port of the engine,and a supercharger having an inlet and an outlet in the charge airchannel to provide compressed charge air to the piston-controlled intakeports of the opposed-piston engine, comprising: a drive unitmechanically coupling the supercharger to the engine; a first gaspressure sensor for generating a first signal indicating a superchargerinlet charge air pressure (P_(in)) in the charge air channel; a secondgas pressure sensor in the charge air channel for generating a secondsignal indicating a supercharger outlet charge air pressure (P_(out)) inthe charge air channel; and, an engine control unit in data signalcommunication with the first and second gas pressure sensors fordiagnosing operation of the supercharger by: determining a firstsupercharger mass flow value based upon the ratio (P_(out)/P_(in));determining a second supercharger mass flow value based upon a value ofmass air flow into the charge air channel; comparing the first andsecond supercharger mass flow values to determine a mass flow comparisonvalue; and, if the mass flow comparison value meets a mass flowperformance measure, correlating the mass flow comparison value with thediagnostic conclusion that the supercharger is operable to providecompressed charge air to the piston-controlled intake ports of theopposed-piston engine; and, providing a control signal to begindiagnosis of one of a supercharger bypass valve and a superchargervariable speed drive; otherwise, correlating the mass flow comparisonvalue with the diagnostic conclusion that the supercharger is notoperable to provide compressed charge air to the piston-controlledintake ports of the opposed-piston engine; and, triggering a monitor tooutput a supercharger fault indication.
 2. A diagnostic system accordingto claim 1, in which the opposed-piston engine further includes asupercharger bypass valve in fluid communication with the charge airchannel downstream of the supercharger outlet and a bypass valveposition sensor, and the engine control unit is operable to diagnoseoperation of the supercharger bypass valve position sensor by:determining a first bypass valve position based upon a valve effectivearea; determining a second bypass valve position provided by the bypassvalve position sensor; comparing the first and second bypass valvepositions to determine a bypass valve position comparison value; and, ifthe bypass valve position comparison value meets a bypass valveperformance measure, correlating the bypass valve position comparisonvalue with the diagnostic conclusion that the bypass valve positionsensor is operable to provide an indication of the bypass valveposition; otherwise, correlating the bypass valve position comparisonvalue with the diagnostic conclusion that the bypass valve positionsensor is not operable to provide an indication of the bypass valveposition; and, triggering the monitor to output a bypass valve positionsensor fault indication.
 3. A diagnostic system according to claim 2 inwhich the engine control unit is further operable to diagnose operationof the supercharger bypass valve by: determining a third bypass valveposition based upon one of a bypass valve control signal and a bypassvalve actuator current; determining a fourth bypass valve position basedupon measurement of the bypass valve position; comparing the third andfourth bypass valve positions to determine a second bypass valveposition comparison value; and, if the second bypass valve positioncomparison value meets a second bypass valve performance measure,correlating the second bypass valve position comparison value with thediagnostic conclusion that the bypass valve is operable to shunt chargeair from the charge air channel; otherwise, correlating the secondbypass valve position comparison value with the diagnostic conclusionthat the bypass valve is not operable to shunt charge air from thecharge air channel; and, triggering the monitor to output a bypass valvefault indication.
 4. A diagnostic system according to claim 1, in whichthe drive unit includes a variable-speed drive coupled to drive thesupercharger from a crankshaft of the engine, and the engine controlunit is operable to detect a fault in operation of the variable-speeddrive by: setting the variable-speed drive to a first drive ratio;determining a first supercharger pressure ratio at the first drive ratiosetting of the variable-speed drive; changing the variable-speed driveto a second drive ratio; determining a second supercharger pressureratio at the second drive ratio setting of the variable-speed drive;comparing the first and second supercharger pressure ratios to determinea supercharger pressure ratio comparison value; and, if the superchargerpressure ratio comparison value meets a supercharger pressure ratioperformance measure, correlating the supercharger pressure ratiocomparison value with the diagnostic conclusion that the variable-speeddrive is operable to drive the supercharger; otherwise, correlating thesupercharger pressure ratio comparison value with the diagnosticconclusion that the variable-speed drive is not operable to drive thesupercharger; and, triggering the monitor to output a variable-speeddrive fault indication.
 5. A diagnostic system according to any one ofclaims 1, 2, 3, or 4, in which the opposed-piston engine furtherincludes an EGR channel and determining the second supercharger massflow value includes combining the value of mass air flow into the chargeair channel with a value of mass exhaust flow in the EGR channel.
 6. Amethod for on-board diagnosis of air handling system faults of anopposed-piston engine including at least one cylinder withpiston-controlled exhaust and intake ports near respective ends of thecylinder, a charge air channel to provide charge air to at least oneintake port of the engine, and a supercharger having an inlet and anoutlet in the charge air channel to provide compressed charge air to thepiston-controlled intake ports of the opposed-piston engine, comprisingthe steps of: mechanically coupling the supercharger to the engine;providing a first gas pressure sensor upstream of the supercharger inletand a second gas pressure sensor downstream of the supercharger outlet;determining a supercharger inlet charge air pressure (P_(in)) in thecharge air channel with the first gas pressure sensor; determining asupercharger outlet charge air pressure (P_(out)) in the charge airchannel with the second gas pressure sensor; determining a firstsupercharger mass flow value based upon the ratio (P_(out)/P_(in));determining a second supercharger mass flow value based upon a value ofmass air flow into the charge air channel; comparing the first andsecond supercharger mass flow values to determine a mass flow comparisonvalue; and, if the mass flow comparison value meets a mass flowperformance measure, correlating the mass flow comparison value with thediagnostic conclusion that the supercharger is operable to providecompressed charge air to the piston-controlled intake ports of theopposed-piston engine; and, providing a control signal to begindiagnosing one of a supercharger bypass valve and a superchargervariable speed drive; otherwise, correlating the mass flow comparisonvalue with the diagnostic conclusion that the supercharger is notoperable to provide compressed charge air to the piston-controlledintake ports of the opposed-piston engine; and, triggering a monitor tooutput a supercharger fault indication.
 7. A method according to claim6, in which the opposed-piston engine further includes a superchargerbypass valve in fluid communication with the charge air channeldownstream of the supercharger outlet and a bypass valve positionsensor, wherein diagnosing a fault in operation of the superchargerbypass valve includes: determining a first bypass valve position basedupon a bypass valve effective area; determining a second bypass valveposition provided by the bypass valve position sensor; comparing thefirst and second bypass valve positions to determine a bypass valveposition comparison value; and, if the bypass valve position comparisonvalue meets a bypass valve performance measure, correlating the bypassvalve position comparison value with the diagnostic conclusion that thebypass valve position sensor is operable to provide an indication of thebypass valve position; otherwise, correlating the bypass valve positioncomparison value with the diagnostic conclusion that the bypass valveposition sensor is not operable to provide an indication of the bypassvalve position; and, triggering the monitor to output a bypass valveposition sensor fault indication.
 8. A method according to claim 7,wherein diagnosing a fault in operation of the supercharger bypass valvefurther includes: determining a third bypass valve position based uponone of a bypass valve position control signal and a bypass valveactuator current; determining a fourth bypass valve position based uponmeasurement of the bypass valve position; comparing the third and fourthbypass valve positions to determine a second bypass valve positioncomparison value; and, if the second bypass valve position comparisonvalue meets a second bypass valve performance measure, correlating thesecond bypass valve position comparison value with the diagnosticconclusion that the bypass valve is operable to shunt charge air fromthe charge air channel; otherwise, correlating the second bypass valveposition comparison value with the diagnostic conclusion that the bypassvalve is not operable to shunt charge air from the charge air channel;and, triggering the monitor to output a bypass valve fault indication.9. A method according to claim 6, in which the engine further includes asupercharger variable speed drive coupled to drive the supercharger fromthe crankshaft, wherein diagnosing a fault in operation of thevariable-speed drive includes: setting the variable-speed drive to afirst drive ratio setting; determining a first supercharger pressureratio at the first drive ratio setting of the variable-speed drive;changing the variable-speed drive to a second drive ratio setting;determining a second supercharger pressure ratio at the second driveratio setting of the variable-speed drive; comparing the first andsecond supercharger pressure ratios to determine a supercharger pressureratio comparison value; and, if the supercharger pressure ratiocomparison value meets a supercharger pressure ratio performancemeasure, correlating the supercharger pressure ratio comparison valuewith the diagnostic conclusion that the variable-speed drive is operableto drive the supercharger; otherwise, correlating the superchargerpressure ratio comparison value with the diagnostic conclusion that thevariable-speed drive is not operable to drive the supercharger; and,triggering the monitor to output a variable-speed drive faultindication.
 10. A method according to any one of claims 6, 7, 8, or 9,in which the opposed-piston engine further includes an EGR channel anddetermining the second supercharger mass flow value includes combiningthe value of mass air flow into the charge air channel with a value ofmass exhaust flow in the EGR channel.
 11. An on-board diagnostic systemof an opposed-piston engine comprising a compressor followed by asupercharger in a two-stage compression configuration coupled to acylinder intake port of the engine, and a supercharger bypass valve tomodulate charge air pressure at the intake port, in which the on-boarddiagnostic system comprises an actuating means for opening and closingthe supercharger bypass valve, a control means responsive to closure ofthe supercharger bypass valve for comparing an estimated mass airflowthrough the supercharger with a measured mass airflow through thesupercharger, and a diagnostic means for (i) evaluating performance ofthe supercharger based on the comparison, and (ii) triggering output ofa supercharger fault indication if the comparison fails to meet a massflow performance measure.